The position of substructures and DNA sequences within the nucleus is highly organized, and this organization plays a role in regulating chromatin-dependent processes, such as gene expression. X chromosome inactivation is an experimentally amenable example of a phenomenon in which regulated changes in the location of specific DNA sequences is implicated in control of gene expression. In mammalian female cells, one X chromosome is silenced for dosage compensation of X-linked genes between males and females, and this process is essential for female survival. Here, we propose to investigate the molecular basis of the regulated changes in X chromosome nuclear organization that accompany X-inactivation. The insights gained are likely to be more generally applicable, since many of the factors that regulate X-inactivation also regulate gene expression in other contexts. PUBLIC HEALTH RELEVANCE: Early in mammalian development, one of the X chromosomes is silenced in each cell, to equalize X- linked gene dosage between XY males and XX females. X-inactivation is essential for survival of mammalian females, and many of the factors that regulate X-inactivation also regulate gene expression in other contexts. We propose to study the molecular basis of changes in X chromosome organization during X- inactivation. This will provide important information about the role of nuclear organization in X- inactivation and will be relevant to understanding how nuclear organization impacts gene expression for the rest of the genome.